It has long been desired to develop a long wave infrared source which is also solid state. Applications for such long wave lasers include infrared countermeasures, and targeting systems as well as biochemical detection using laser spectroscopy. In order to provide long wave infrared radiation, one starts at a moderately short wavelength of about 2 microns. It is then necessary to be able to convert the 2 micron pumping energy to the 8 to 12 micron band.
Intrinsically this is not a very efficient process because one is starting with 2 micron photons and trying to obtain 8 microns. As a result there is a very large quantum defect, relating to the difference in wavelength between the pumping radiation and the desired output. The typical way of addressing the quantum defect problem is to utilize an optical parametric oscillator which is a non-linear optical device in which one pumps the non-linear optical crystal in the oscillator at one wavelength to obtain an output at another wavelength.
One type of non-linear optical crystal is a zinc germanium phosphide crystal which when pumped at around 2 microns using a Holmium laser results in a non-linear process, called a parametric process, where one creates pump photons which split into two photons. With conservation of energy, the two photons split up into energies that add up to that which is equal to the energy of the pump photon. However for this to occur, one needs phase matching. Phase matching occurs where one has indices in the crystal for the various waves and are such that all the waves can grow together in phase. If one does not have that condition, then what happens is that the generated waves convert back into the pump wavelength and one does not obtain much generation.
However, if one can get into a condition where the waves are in phase, the waves continue to grow. Thus it is an important part of the process to have phase matching.
Once one has phase matching for the wavelengths of interest in the non-linear crystal, one can enclose the crystal in an optical resonator which then allows waves to build from noise and to do so more efficiently than it would if one where going single pass through the crystal.
One can resonate either the signal wavelength or the idler wavelength. If only one wavelength is resonated it is commonly called singly resonated or the optical parametric amplifier is called a singly resonant optical parametric oscillator, SRO. With an optical parametric oscillator one can resonate both waves, both the signal and idler, and get what is called a doubly resonant oscillator. However, this does not turn out to be advantageous because it is very unstable. However, it does result in a much lower lasing threshold.
Referring now to the use of singly resonant oscillators, if one utilizes a single crystal pumped at for instance 2 microns and one wants to shift the output to 8 microns, one normally obtains the shifted signal in a parametric process. In such a process one obtains perhaps 50% conversion efficiency, meaning the power conversion efficiency from pump to signal and idler combined. If one then factors in the quantum defect, one is only obtaining 10% of the total pump power out in the long wave 8 micron infrared output.
The result is that a significant amount of power is not being used because it is going into the signal as opposed to the idler. In one example, a signal wavelength may, for instance, be at 2.8 microns. However, the signal is not contributing to the process and in general it simply escapes out to the rest of the world.
The result is that the idler signal comes out of the optical parametric amplifier at only a tenth of the input pump power.
In order to solve the problem of efficiency, in the past single optical parametric oscillators called tandem optical parametric oscillators have been cascaded. This results in two discrete optical parametric oscillators, each in a separate cavity. The disadvantage is that it is a complex system because one has multiple resonant cavities. Secondly, efficiency of the type desired cannot be met due to the wastage of photons, namely signal photons to which the cavities are not tuned and which do not contribute to the idler.